1. Field of the Invention
The present invention relates to a motor drive control device for driving and controlling at least one servo motor configured to drive one feed axis to move a table or tool of a machine tool and one spindle motor having one main axis to rotate a table or tool of a machine tool.
2. Description of Related Art
In such a motor drive control device, in order to drive and control a servo motor and a spindle motor, alternating-current voltage or alternating current from a commercial alternating-current power source, such as a three-phase alternating-current power source, is converted into direct-current voltage or direct current and then, converted into alternating-current voltage or alternating current of an arbitrary frequency and the converted alternating-current voltage or alternating current is supplied to the servo motor and the spindle motor, respectively. To do this, the motor drive control device has: a first inverter unit to which one rectifier unit configured to convert alternating-current voltage or alternating current into direct-current voltage or direct current is connected, and which is configured to convert the direct-current voltage or direct current supplied from the rectifier unit into alternating-current voltage or alternating current and to supply the converted alternating-current voltage or alternating current to the servo motor; and a second inverter unit to which the rectifier unit is connected and which is configured to convert the direct-current voltage or direct current supplied from the rectifier unit into alternating-current voltage or alternating current and to supply the converted alternating-current voltage or alternating current to the spindle motor.
If the electric power, which can be supplied by the rectifier unit, larger than the sum of the maximum output of the servo motor and the maximum output of the spindle motor is set, there is no possibility of the output of the servo motor and the output of the spindle motor running short. However, it is rare that the servo motor and the spindle motor reach the maximum output at the same time, and therefore, the electric power, which can be supplied by the rectifier unit, is usually set to an output supposed to be smaller than the sum of the maximum output of the servo motor and the maximum output of the spindle motor from the viewpoint of the installation space and cost of the rectifier unit.
On the other hand, in the case where the sum of the output of the servo motor and the output of the spindle motor exceeds the electric power, which can be supplied by the rectifier unit, because of a high load caused during a machining operation, such as a cutting operation of a workpiece by a machine tool, etc., an alarm is issued and the machining operation is stopped by emergency-stopping the rectifier unit.
In particular, if an instantaneous input power drop etc., occurs, the electric power, which can be supplied from the rectifier unit, is reduced, however, in order to prevent the machining operation from being stopped even in such a case, a motor drive control device that limits the torque of a motor so that the sum of the outputs of the motors does not exceed the electric power, which can be supplied by the rectifier unit, is proposed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 2002-354396 (JP2002-354396A) and WO2008-093485 (JP2008-093485A1)
If the torque of the servo motor is limited in order to limit the sum of the output of the servo motor and the output of the spindle motor, the ratio of the output of the servo motor to the sum of the output of the servo motor and the output of the spindle motor is remarkably small (for example, 10%), and therefore, it is difficult to effectively limit the sum of the output of the servo motor and the output of the spindle motor.
Further, if the torque of the servo motor is limited in order to limit the sum of the output of the servo motor and the output of the spindle motor, there may be a case where the cutting operation of a workpiece by the machine tool is adversely affected because of deterioration of tracking of the position command to the servo motor.
On the other hand, if the torque of the spindle motor is limited in order to limit the sum of the output of the servo motor and the output of the spindle motor, there may be a case where the cutting operation of a workpiece by the machine tool is affected adversely when it is no longer possible to continue the cutting operation because of the stop of the spindle motor due to the shortage of torque during the cutting operation of a workpiece by the machine tool, in particular, during the cutting operation of a heavy workpiece.